The complex made by treating commercial, purified DNA polymerase I with factor "E" is the same as DNA-synthesizing complex found naturally insofar as its electrophoretic mobility in detergent and non-detergent gel systems, its elution volume on BioGel A-1.5m columns, its reactivity with native DNA and its stimulation by ATP. Factor E, retained by the membrane fraction after complex has been released by EDTA treatment, is released by sonication but it is still excluded from a BioGel A-50m column and is sedimentable at 100,000 xg. The E obtained after these procedures is freer from contaminating activities of DNase and polymerase, both of which are mostly non-sedimentable. DOC extraction of the EDTA-treated membrane produces a from of active E which is included in a BioGel A-1.5m column. An ATP-Sepharose column will bind complex only if Mg-2+ (10 mM) is present. The bound complex is then eluted with ATP. As in all previous manipulations involving complex, recovery of activity is dependent on the presence of factor E.